1. Field of the Invention
The present invention relates to a semiconductor device, for example, to an insulating gate type field effect transistor using a nitride semiconductor.
2. Related Background Art
Since a nitride semiconductor device using gallium nitride (hereinafter referred to simply as GaN) has a large band gap as compared with a semiconductor device using silicon (Si), the semiconductor device has a high critical electric field, and from this characteristic, a small-sized device having a high breakdown voltage is easily realized. Accordingly, in a semiconductor for electric power control, a low on-resistance is achieved, and a device having a low loss can be realized. Above all, in a field effect transistor using an AlGaN/GaN heterostructure (hereinafter referred to simply as an HFET (a heterostructure field effect transistor), satisfactory characteristics can be expected with a simple device structure. A gate electrode in AN HFET has a Schottky gate structure forming a Schottky junction with an AlGaN layer. Moreover, a conventional GaN-based HFET is a normally on-type device in which a current flows between a source and a drain, when a drain voltage is applied at a gate voltage of zero.
However, in general, the Schottky gate structure has a problem that a leak current increases, when a gate leak current is large and device temperature rises. A normally-on type device has a problem that a large current flows at the moment at which a power to a circuit is turned on, and this sometimes results in destruction of the device.